libavif: add libyuv as dependencies
libavif: add libyuv as dependencies

libyuv could be used to optimize some operations,
applications use libavif may slightly benefit from that.

A trivial benchmark:

(Sample picture is converted with `avifenc pic.png pic.avif`)

- without libyuv
```
$ hyperfine --warmup 4 'avifdec pic.avif /tmp/pic.png'
Benchmark 1: avifdec pic.avif /tmp/pic.png
  Time (mean ± σ):     641.4 ms ±   2.5 ms    [User: 631.9 ms, System: 9.1 ms]
  Range (min … max):   637.7 ms … 645.3 ms    10 runs
```

- with libyuv
```
$ hyperfine --warmup 4 './result/bin/avifdec pic.avif /tmp/pic.png'
Benchmark 1: ./result/bin/avifdec pic.avif /tmp/pic.png
  Time (mean ± σ):     627.5 ms ±   4.6 ms    [User: 618.0 ms, System: 9.0 ms]
  Range (min … max):   622.4 ms … 635.9 ms    10 runs
```

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